Aims
The aims of the present invention are to:                1. Increase the efficiency of wave energy conversion by dynamically tuning an oscillating buoyant body immersed in the ocean to achieve resonance with energetic ocean swell over the range of periods that characterize such swell.        2. Reduce risk of damage in severe sea states by dynamically tuning an oscillating buoyant body to reduce resonance with ocean swell.        3. Achieve the required dynamic tuning by varying continuously and rapidly the inertial mass of the buoyant body.        4. Vary the inertial mass of the buoyant body without major energy consumption, as required, for example, when pumping water out of a flooded buoyant body.        5. Vary the inertial mass of the buoyant body without incurring significant drag losses.        6. Vary the inertial mass of the buoyant body without incurring a fixed overhead of added mass sufficiently large to make wide-range tuning difficult.Dynamic Tuning        
Dynamic tuning means here the continuous real-time control of the period of oscillation of a buoyant body so that this period matches the varying period of the dominant energetic ocean swell.
Ocean swell is generated by mid-ocean storms, and as the swell propagates, the longer period components travel faster, so that successive swell trains are formed with successively shorter periods. In a simplified example, a mid-Atlantic storm instantly propagates swell to the Irish coast over a distance of 2000 km. A swell train of 20 seconds period travels at 56 kph, arriving at the coast after 36 hours. A swell train of 10 seconds period arrives after 72 hours. In this case, the average tuning speed required is 10/36=0.3 seconds per hour. In practice, swell is not instantly propagated but requires persistent high winds over large areas of ocean for a sufficiently high sea to develop. The swell energy per meter of wave front varies with the square of the swell height and with the speed of the swell, which in turn varies with the period. The most intense and persistent storms generate the highest swell with the longest period: usually not more than 25 seconds.
The scale of a storm, its duration and its distance can all vary, so that the average required tuning rate can be as high as 1 second per hour. Faster tuning can be required when the short period swells of a fading storm are succeeded by the long period swells of a new storm. For example, a 10 second swell may be succeeded by a 20 second swell in less than three hours, implying a tuning rate of over 3 seconds per hour. But the fraction of the year during which swell of 20 seconds or more occurs is typically small and the fraction of time over which rapid upward change in swell period occurs is also small. It may therefore not be economic to engineer a wave energy converter (WEC) to be tunable at this extreme rate.
The maximum design rate of tuning of a WEC depends on the balance of gain and cost. The gain is incrementally improved energy transfer. The cost depends on the method of tuning. For example, if the tuning method involves pumping water in and out of the buoyant body, then there are significant associated capital, operating and maintenance costs. The present invention describes a tuning method that enables precise tuning at rates of up to several seconds per hour at costs substantially lower than required for tuning by pumping.
Tuning Range
In general, at a given location, swells of different periods will arrive from different directions from storms at different distances. Swells of greater than 10 seconds period can travel thousands of km in deep water with only slight frictional attenuation, although all swells are subject to reducing wave height as the wave front spreads. Swells with shorter periods attenuate more rapidly. Swells pass through each other, so that in general it is possible to identify a dominant swell and tune a buoyant body to the dominant swell. Tuning can be assisted if the direction of oscillation of the buoyant body can be oriented to the dominant swell.
The frequency distribution of ocean swell energy peaks at 6-8 seconds period in the absence of significant storms, rising to around 13-15 seconds period for major oceanic storms, involving winds over 50 kph over thousands of square kilometers of ocean, acting over several days. Analysis of over 140,000 measurements over six years at ten widely separated locations across the North Atlantic showed that swells with periods exceeding 15 seconds occur less than 0.4% of the time. Swells with periods under 5 seconds occur 25% of the time in mid-Atlantic but on the west coast of Ireland and Scotland occur, as a result of attenuation, only 6% of the time. Because swell energy varies with swell period, the proportion of total annual swell energy below 5 seconds period in the latter locations is only a few %. To capture most of the available annual swell energy requires a dynamic tuning range between 5 and 15 seconds ie a 3× variation in period. This is referred to as wide-range dynamic tuning. Depending on the typical sea states at the location of the buoyant body the required range could be, for example, 6 to 18 seconds or 7 to 21 seconds. A 3× variation in period can be achieved with a 9× variation in total mass moment of inertia. The present invention describes how to achieve a sufficiently large variation in inertial mass to achieve a 3× variation in period.
Wave Energy Conversion Efficiency
A WEC converts wave motion to the motion of a buoyant body (usually floating on the water surface but sometimes buoyant and submerged) and uses the relative motion of the floating body and a reference body to move a conductor through a magnetic field, so generating an electric current. The electrical generator can be in situ or remote (for example, the WEC pumps water to a shore station to drive a turbine). The reference body can be a fixed platform such as a wind turbine plinth, or a pseudo-stationary body that has large inertial mass, or a second floating body that is arranged to be out of phase, or an inertial device. In the case of an oscillating water column WEC, the relative motion is between a trapped column of water and the enclosing structure. There can be many inefficiencies in this process. One of the largest sources of inefficiency is a mismatch between the natural frequency of oscillation of the buoyant body and the frequency of the impinging wave. Where the sea state is effectively random, this is unavoidable. However, in many locations on the edges of large oceans, for example, off the west coast of Scotland, there are predictable energetic swells. In such locations, a common strategy is to design the WEC with a single natural frequency that matches the average annual local peak energy frequency or to design the WEC with slow tuning over a limited range of frequencies, for example, using variable ballast to match seasonal sea states. However, this is a poor substitute for wide-range dynamic tuning.
Varying the Inertial Mass
It is possible to tune an oscillating floating body by pumping water in and out of it. But significant amounts of energy and time can be required to pump the water. Also pumped-in water displaces air, so changing the displacement and balance of the floating body. An alternative is to contain water in one or more submerged vessels that are attached to an oscillating floating body. When the vessel is open in the direction of oscillation—for example, the vessel is a cylinder that is open at both ends—the contained water is not ‘trapped’ and has no significant inertial effect. When such a cylinder is closed at both ends, the contained water is ‘trapped’ and it has an inertial effect and so varies the period of oscillation of the floating body. There is no pumping. The vessel can be opened and closed rapidly: for example, in a few seconds. In this manner, the period of oscillation of the buoyant body can be changed rapidly. This method is called ‘inertial trapping.’
Provided the open and closed vessels are streamlined and have a fineness ratio exceeding around three, there is negligible added mass and energy losses due to drag and friction are small. If the vessel geometry is varied so that the fineness ratio falls below three, then the added mass rises. When the fineness ratio equals one, the streamlined vessel is spherical and the added mass is around 50% of the mass of the contained water. This can be useful in that the variability of total mass (ie mass plus added mass, sometimes called virtual mass) is increased and so the range of tuning is extended. The drawback is that drag is also increased. The design decision on the minimum allowed fineness ratio depends on a balance between the advantages of wider and faster tuning and the disadvantage of drag losses. Across most of the range of variation in vessel geometry it is preferable to have minimal drag losses ie a vessel should preferably have a smooth lenticular cross-section in the oscillating vector. Such a lenticular profile also incurs very little added mass. Low added mass at low volumes of contained water assists the variability in total mass ie the tuning range.
Gregory in GB1218866.0 describes tuning of a heaving floating body by such inertial trapping. Attached to a heaving floating body is a plurality of ‘inertial traps’, comprising submerged vertical tubes of different volumes. By opening and closing these tubes in different combinations, it is possible to trap different inertial masses of water, for example, varying the total amount trapped nine-fold. Provided the fixed overhead of mass and added mass is kept low, variation in the mass of contained water is sufficient to enable wide range dynamic tuning. But these traps have deficiencies:                The traps are kept streamlined at all stages of operation, in an example, by using spindle-shaped plugs to open and close the tubes. But such plugs have a fixed mass that reduces the available range of variation in total mass.        The tuning is in incremental steps and the increments can be reduced only by increasing the number of traps.        Multiple traps are required.        
The present invention solves these problems, with traps that:                Can be opened and closed without significant additional fixed mass and with streamlining maintained.        Permit continuous fine variation in trapped volume.        Enable a single trap to assist dynamic tuning of a floating body.Other Prior Art        
Noren in U.S. Pat. No. 4,773,221 describes a wave energy converter that captures the relative motion of a buoyant body and a submerged, vertical, open-ended tube. The buoyant body is connected to a piston that slides inside the tube. Tuning is not claimed. Lack of streamlining means that fixed added mass and drag losses are significant.
Dick in US 20100034588 describes a WEC comprising a surface-piercing float linked by a power take off to a streamlined submerged body having an entrapped volume of seawater. The difference in the oscillation frequency of the float and the submerged body enables power capture. The submerged body includes at least one cylindrical compartment that can be opened or closed at either end to trap or release water. The deficiencies of this design are:                The described trapping of water does not tune the float to resonate with the dominant swell, so improving energy capture. Instead it ‘detunes’ a submerged body against which the float reacts.        The submerged body ‘includes’ a compartment ‘sealed off from the remaining portion of the body’ ie the submerged body has a significant fixed inertial mass, so that tuning is limited to a narrow range.        The compartment for trapping water is a cylinder of fixed dimensions. Tuning using variable geometry is not described.        The means of closing the compartment is not specified to ensure streamlining, so that the added mass can significantly affect tuning and drag losses can be substantial.        
Fraser et al in U.S. Pat. No. 7,726,123 describes a WEC comprising a floating vessel with an array of vertical chambers extending to different depths so as to provide a range of resonant frequencies of the air columns above the water level. These air columns drive a turbine. The inertia of the entrained water affects the resonant frequency but the chambers have fixed geometries and so fine tuning is not possible.
Stewart et al in US2007/0046027 describes a WEC with flexible or telescoping floats that can be inflated. The purpose is not to tune the floats but to deploy them.
Burns in US20100171312 describes a WEC using a buoyant actuator with a pliant outer skin and a mix of solid and fluid contents that jointly allow variation in buoyancy and response area. This differs from the present invention, which describes a non-buoyant device that contains only a variable volume of water, that is always streamlined in the direction of oscillation and that varies the inertial mass only.
Rohrer in US 2010/0308590 describes a WEC using variable geometry containers but these are for capturing motion, are not used in tuning and are not streamlined in the direction of oscillation. Rohrer references several other similar designs.